The present invention relates to a device for extruding honeycomb structures which are used as catalyst supports, heat exchangers or filters.
Conventionally, the mechanical strength of the whole of the ceramic honeycomb structure or of the outer peripheral portion thereof has been improved by forming an outer wall B which is thicker than a grid-shaped wall A, in the outer periphery of the honeycomb structure as shown in FIG. 1.
For extruding such a honeycomb structure as described above, devices such as one shown in FIG. 2 are conventionally used.
The conventional device is provided with a die mask 1 having an inner surface 10 of a smaller diameter than an extrusion die 3, in the outer periphery of the outlet side of the extrusion die 3 disposed within a cylinder 2 respectively.
The extrudable material which was supplied from the cylinder 2 passes feed passageways 4 and grid-shaped extrusion slots 5 which are formed within the die 3, under pressure to be extruded therefrom.
Then, the extruded body is formed by the inner wall 10 of the die mask 1 into a predetermined outer form and the thick outer wall is also formed.
However, according to the device of FIG. 2, the material is pressed by the tapered inner wall 10 of the die mask 1 toward the center thereof after being extruded out of the die 3. Therefore, distortion is easy to occur in the connecting portion C between the grid-shaped wall A and the outer wall B of the obtained honeycomb structure as shown in FIG. 3. As a result, the mechanical strength of the connecting portion C is decreased.
Furthermore, since the thickness of the outer wall of the honeycomb structure is largely different from that of the grid-shaped wall thereof, the heat transfer from the grid-shaped wall portion into the outer wall becomes discontinuous in the connecting portion therebetween, or the inclination of the heat transfer abruptly changes therein when the conventional honeycomb structure is used in the exhaust gas purifying device of an automobile. As a result, the honeycomb structure is easy to be broken in the connecting portion.
These drawbacks of the honeycomb structure are inevitable when it is extruded by the device shown in FIG. 2. And these drawbacks are caused by the defects of the conventional extrusion device itself.
There has previously been proposed one honeycomb structure in which the wall thickness is increased continuously or by stages from the central portion to the outer peripheral portion thereof in U.S. Pat. No. 4,233,351 assigned to the assignee hereof by inventors Okumura et al.
In this patented honeycomb structure, by gradually increasing the wall thickness thereof in the direction of the outer periphery thereof, thermal inclination becomes gentle and excellent thermal shock resistance can be obtained. And since the wall thickness of the outer peripheral portion is large, excellent mechanical strength can be obtained.
When the honeycomb structure is used as a catalyst support to be disposed in the exhaust system of a vehicle, the honeycomb structure is retained within a casing. In this case, it is required to maintain air-tight condition good between the outer peripheral surface of the honeycomb structure and the inner surface of the casing in order that the exhaust gas does not leak therethrough. Therefore, it is desired to make the outer peripheral surface of the honeycomb structure smooth.
Accordingly, one object of the present invention is to provide an extrusion device for producing honeycomb structures having an excellent mechanical strength and thermal shock resistance.
Another object of the present invention is to provide an extrusion device for producing honeycomb structures of which wall thickness is increased from the central portion toward the outer peripheral portion thereof with uniform density without generating any distortion in the wall thereof.
Still another object of the present invention is to provide an extrusion device for producing honeycomb structures having smooth outer peripheral surfaces.